Phenol is an important product in the chemical industry and is useful in, for example, the production of phenolic resins, bisphenol A, ε-caprolactam, adipic acid, and plasticizers.
Currently, the most common route for the production of phenol is the Hock process. This is a three-step process in which the first step involves alkylation of benzene with propylene to produce cumene, followed by oxidation of the cumene to the corresponding hydroperoxide, and then cleavage of the hydroperoxide to produce equimolar amounts of phenol and acetone. However, the world demand for phenol is growing more rapidly than that for acetone. In addition, due to a developing shortage, the cost of propylene is likely to increase.
Thus, a process that uses higher alkenes instead of propylene as feed and coproduces higher ketones, such as cyclohexanone, rather than acetone may be an attractive alternative route to the production of phenols. For example, there is a growing market for cyclohexanone, which is used as an industrial solvent, as an activator in oxidation reactions and in the production of adipic acid, cyclohexanone resins, cyclohexanone oxime, caprolactam, and nylon 6.
It is known from, for example, U.S. Pat. No. 6,037,513 that cyclohexylbenzene can be produced by contacting benzene with hydrogen in the presence of a bifunctional catalyst comprising a molecular sieve of the MCM-22 family and at least one hydrogenation metal selected from palladium, ruthenium, nickel, cobalt, and mixtures thereof. The contacting is conducted at a temperature of about 50° C. to 350° C., a pressure of about 100 kPa to 7000 kPa, a benzene to hydrogen molar ratio of about 0.01 to 100 and a WHSV of about 0.01 to 100. The '513 patent also discloses that the resultant cyclohexylbenzene can be oxidized to the corresponding hydroperoxide which is then decomposed to the desired phenol and cyclohexanone co-product.
However, although benzene hydroalkylation is an attractive route for the production of cyclohexylbenzene, with current processes the selectivity to the desired cyclohexylbenzene product becomes undesirably low at conversions much above 30%. In addition, the hydroalkylation of benzene over an acid catalyst, such as MCM-22, is highly exothermic and high temperatures are an important factor leading to reduced cyclohexylbenzene selectivity. The major impurities in the product are cyclohexane and dicyclohexylbenzene. Cyclohexane builds up in the C6 recycle streams and must be removed by treatment or purging, whereas the dicyclohexylbenzene by-product requires transalkylation. Although transalkylation of dicyclohexylbenzene with benzene produces additional cyclohexylbenzene product, the cost of transalkylation is not insignificant. There is therefore a need to provide a benzene hydroalkylation process which provides improved selectivity to monocyclohexylbenzene at acceptable benzene conversion rates.
U.S. Pat. No. 3,784,617 discloses a process for the hydroalkylation of mononuclear aromatic compounds, in which an aromatic charge, such as benzene, and a first portion of hydrogen are reacted in a first stage to produce a partially hydroalkylated stream and, after cooling, the partially hydroalkylated stream and a second portion of hydrogen are reacted in a second stage to produce a hydroalkylate product. Introducing the hydrogen in multiple stages reduces the degree of benzene conversion, and hence the temperature rise, in each stage. By avoiding excessive temperature increases, more favorable product selectivities are said to be obtained.
U.S. Patent Publication No. 2010/0317895 discloses a process for producing cyclohexylbenzene, the process comprising: (a) introducing hydrogen and a liquid feed comprising benzene into a reaction zone; (b) contacting the benzene with the hydrogen in the reaction zone under hydroalkylation conditions to produce cyclohexylbenzene; (c) removing an effluent stream comprising cyclohexylbenzene and unreacted benzene from said reaction zone; (d) dividing the effluent stream into at least first and second portions; (e) cooling the effluent stream first portion; and (f) recycling the cooled effluent stream first portion to the reaction zone, wherein the ratio of the mass of the effluent stream first portion to the mass of effluent stream second portion is at least 2:1. At least part of the effluent stream second portion may be cooled and fed to at least one further reaction zone where the unreacted benzene in the effluent stream second portion is contacted with further hydrogen under hydroalkylation conditions to produce further cyclohexylbenzene.
The present invention seeks to provide improved designs for providing temperature control of the reagents in benzene hydroalkylation processes and particularly those employing staged addition of the hydrogen.